In SLS machine concepts. EOS focused all

 

In recent decades conventional manufacturing processes
like injection molding for mass production was developed. The correlating
construction limitations are commonly known and published many times. The
industrial designers and the engineers are trained for this conventional
manufacturing technique to make the parts suitable for production and to
consider tool related restrictions like how to take the parts out of the mold
or how to prevent sharp edges.

 

Increasing competitive pressure, shorter product
lifecycles, demand for individual products and low running resources are
increasing the search for convenient and innovative manufacturing methods rapidly.
The recent advancements of Rapid Prototyping (RP) and Rapid
Manufacturing (RM) are offering the Additive Manufacturing (AM) processes to
get involved in small-series production of customized products, and
simultaneously tool-free production is providing high design freedom for
components. Basics of fabrication with AM starts with designing a part with 3D
computer aided design (CAD), where the data is converted to STL-Format. In a
system-specific slicing software, the STL data is positioned and sliced into
layers, in order that the AM system could process. (1) (2)

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 Selective laser sintering (SLS) is one of the
first commercialized additive manufacturing processes, developed at the
University of Texas by end of eighties. By 1994, the company Elektro Optical
Systems (EOS) introduced their own SLS machine concepts. EOS focused all the
way from the beginning on the technical development, and today considered as world
market leader among the powder-based AM machines producers (3).

Figure 1: Schematic of the
Selective Laser Sintering process (6)
 
Figure 2: Schematic of the
Selective Laser Sintering process (6)

SLS is considered as most popular Rapid Prototyping technique due to
availability of various powder materials for the fabrication and its ability to
build objects without support structures (4). The word
”sintering” is a definition of a process by which objects are built from
powder material via atomic diffusion. The difference between sintering and
melting is that the material never turns completely into liquid phase by
sintering (5). Main working
principal of SLS is sintering a plastic powder layer (mostly 0.1 mm) at
selected areas with a CO2-laser heat source. First, the build
platform is moved down in the z-axis to make room for first layer of powder.
Surface of the powder is heated to barely under melting temperature, thereby
the laser can just scan through and fuse the powder particles together. Navigation
of the laser beam is done by exact positioning of mirrors. As soon as the first
layer is completed, the platform moves down one layer thickness lower more. New
layer of powder is spread by recoater and this time the laser fuses the particles
with the underlying layer. The building process is repeating the same four
steps until the part is layer-by-layer built: (6) (7)

 

 

1.      
Sinking of the build platform one layer
thickness lower

2.      
Laying the new powder on the platform

3.      
Heating the powder bed to process temperature

4.      
Scanning the cross-sections with laser beam (3)

 

 

 

 

Figure 3: EOS FORMIGA P 110
(Source: EOS)
 
Figure 4: EOS FORMIGA P 110
(Source: EOS)

 

 

 

 

 

 

 

 

 

 

Figure 5: Spreading new layer
of powder on lowered platform

Figure 6: Sintered sections on
currently building layer
 
Figure 7: Sintered sections on
currently building layer

1.1      Problem
Statement / Research Questions

 

There are lot of process parameters affecting
the product quality with SLS. The prime problem is that there is not enough
information about outcomes under combination of different settings of
parameters. Variety of useful design guidelines do not exist so far. As an
example for hinges, powder removal by post-processing should have considered at
design phase, otherwise, in some forms it can be impossible to remove the
powder stuck between bush and bolt. Part orientation inside of the building
chamber have major effects on the outcome as well, because of the mechanics of
the laser, angle between laser beam and part surface doesn’t always allow
scanning of perfect circles. Deformation of already fabricated parts by time
because of the humidity is also a known problem with the material Polyamide 12
(PA12), which is the most used material in market. Taking all of these problems
in consideration is leading to these questions, which are going to be
researched in this work:

 

1.      
What is the minimum possible
clearance between bush and bolt when producing hinges?

2.      
What is the effect of part
orientation on hinge clearances?

3.      
What is the effect of humidity
on hinge clearances?

 

1.2      Objective

Aim of this work is finding answers to the research
question given above by practical experiments and create guidelines for hinge fabrication
with SLS machine EOS Formiga P 110. The information is intended to be used as
design guidelines for the best outcome and to minimize errors.

 

The feature demonstrated in this thesis is minimum gap
between bush and bolt for a functional hinge under following factors:

 

·        
Vertical, horizontal and diagonal build
orientation

·        
Clearances with and without humidity deformation

 

All of the listed tests are sintered using SLS machine
EOS Formiga P 110, the parts are manufactured using default setting on the
machine. The material used in these parts was PA12. The result are only valid
for this AM system using this setting, and most likely would not correspond with
other RP systems.

 

 

 

 

1.3      Methodology

 

First chapter of this thesis consist of theory research,
which is explaining the reasons why stated problems are existing. The second
chapter shows how the test pieces are designed and the experiments implemented.
The maximal and minimal dimensions are based on previous experiences. In the last
chapter, theory and practical results are compared and discussed.

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